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Device Selection Drivers

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As the op amp design process is begun, it is useful to keep in mind the fact that there are several selection drivers, which can dictate priorities. This is illustrated by Figure 1-11.
Actually, any single heading along the top of this chart can in fact be the dominant selection driver, and take precedence over all of the others. In the early days of op amp design, when such things as supply range, package type etc. were fairly narrow in spread, performance was usually the major driver. Of course, it is still very much so, and will always be. But, today’s systems are much more compact and lower in power, so things like package type, size, supply range, and multiple devices can often be major drivers of selection. As one example, if the only available supply voltage is 3V, you look at 3V compatible devices first, and then fill other performance parameters as you can.
Some op amp selection drivers
Figure 1-11: Some op amp selection drivers
As another example, one coming from another perspective, sometimes all-out performance can drive everything else. An ultra low, non-negotiable input current requirement can drive not only the type of amplifier, but also its package (a FET input device in a glass-sealed hermetic package may be optimum). Then, everything else follows from there. Similarly, high power output may demand a package capable of several watts dissipation, in which case you find the power handling device and package first, and then proceed accordingly.
At this point, the concept of these "selection drivers" is still quite general. The following sections of the chapter introduce device types, which supplements this with further details of a realistic selection process.
The previous section examined op amps without regard to their internal circuitry. In this section the two basic op amp topologies—voltage feedback (VFB) and current feedback (CFB)— are discussed in more detail, leading up to a detailed discussion of the actual circuit structures in Section 1-3.
Voltage feedback (VFB) op amp
Figure 1-12: Voltage feedback (VFB) op amp
Although not explicitly stated, the previous section focused on the voltage feedback op amp and the related equations. In order to reiterate, the basic voltage feedback op amp is repeated here in Figure 1-12 above (without the feedback network) and in Figure 1-13 below (with the feedback network).
Voltage feedback op amp with feedback network connected
Figure 1-13: Voltage feedback op amp with feedback network connected
It is important to note that the error signal developed because of the feedback network and the finite open-loop gain A(s) is in fact a small voltage, v.
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